Are Planetary Systems Filled to Capacity?

Although the solar system was long regarded as the archetype of clockwork regularity, modern numerical simulations suggest that the orbits of the planets are vulnerable to chaotic variations over very long time scales. Furthermore, if one tried to squeeze another planet in between those that exist, the result would likely be dynamically unstable. The solar system appears to be filled nearly to capacity. Astronomers surmise that it probably began with more planets and that some of these primordial worlds were, one by one, flung off into the Galaxy, leaving today just eight planets in a state of marginal stability. Future observations may reveal whether other planetary systems generally display the same quality.

This article is published in cooperation with NASA’s online Astrobiology Magazine (www.astrobio.net).